1. Field of the Invention
The present invention relates to wide bandwidth hybrid mode feeds and, more particularly, to hybrid mode feeds which are capable of handling very wide bandwidths and include an arrangement which converts a dominant TE.sub.11 mode at the input to the feed into the HE.sub.11 hybrid mode, which hybrid mode is then propagated further or launched into free space.
2. Description of the Prior Art
An important consideration in designing antennas for terrestrial radio relay and satellite communication is excellent radiation characteristics and very low return loss. In this regard the horn reflector is an excellent antenna, but its metal walls are generally uncorrugated. The horn antenna could be improved with corrugations but generally corrugated structures, especially in the size of the horn reflector, are very difficult and expensive to produce. Additionally, the -40DB return loss over a very wide range of frequencies as found with the present uncorrugated horn reflectors is generally not obtainable with the present corrugated feeds.
U.S. Pat. No. 4,040,061 issued to C. G. Roberts et al on Aug. 2, 1977 describes a corrugated horn antenna allegedly having a useful operating bandwidth of at least 2.25:1. There, the antenna is fed with a waveguide in which a TM.sub.11 mode suppressor is disposed in a circular waveguide section before the input wavefront encounters a flared corrugated horn. The mode suppressor functions to prevent the excitation of hybrid modes in the horn at the upper end of a wide band of frequencies which would cause an unacceptable deterioration in the radiation pattern.
U.S. Pat. No. 4,021,814 issued to J. L. Kerr on May 3, 1977 relates to a broad-band corrugated horn antenna with a double-ridged circular waveguide feed allegedly having a bandwidth handling capability greater than 2:1 without the introduction of lossy materials or resistive type mode suppressors. There, a plurality of ridges, each having a predetermined width, and a plurality of gaps between the ridges, with each gap having a predetermined width, are provided wherein the width of the gaps is greater than the width of the ridges.
It has been found that for a waveguide with finite surface impedances, the fundamental HE.sub.11 mode approaches, under certain conditions the behavior that the field essentially vanishes at the boundary and the field is essentially polarized in one direction. Because of these properties, such a mode is useful for long distance communication since it is little affected by wall imperfections or wall losses and provides an ideal illumination for a feed for reflector antennas. In general, it is difficult to excite the HE.sub.11 mode in a corrugated feed since, at the input, the feed is usually excited by the TE.sub.11 mode of a circular waveguide with smooth metal walls. For the TE.sub.11 mode, the transverse wavenumber, .sigma., is related to the waveguide radius by .sigma.a=1.84184. At the feed aperture, however, for the desired HE.sub.11 mode, .sigma.a.perspectiveto.2.4048. Thus the mode parameter u=.sigma.a must increase from 1.84184 to about 2.404 as the mode propagates from the input of the feed to the aperture.
In a corrugated waveguide, u is known to be a decreasing function of the corrugations depth d. Therefore, in order for u to increase, d must decrease in the direction of propagation. To satisfy this requirement, corrugated feeds are usually designed as shown in FIGS. 1 and 2a of U.S. Pat. No. 3,618,106 issued to G. H. Bryant on Nov. 2, 1971. In this regard, see also the articles "Reflection, Transmission and Mode Conversion in a Corrugated Feed" by C. Dragone in BSTJ, Vol. 56, No. 6, July-August 1977 at pp. 835-867 and "Characteristics of a Broadband Microwave Corrugated Feed: A Comparison Between Theory and Experiment" by C. Dragone in BSTJ, Vol. 56, No. 6, July-August 1977, at pp. 869-888. In such arrangement, the input discontinuity of d causes a reflection which vanishes at the frequency satisfying .lambda..sub.r .perspectiveto.2d, where .lambda..sub.r is the wavelength in the radial lines of the input corrugations. The feed can thus be used effectively only in the vicinity of this frequency and, as a consequence, bandwidths in excess of 100 percent are difficult to obtain.
Other arrangements for transforming the TE.sub.11 mode into the HE.sub.11 mode, for subsequent launch from a feed, using helically wound wire structures bonded to the interior surface of a waveguide are disclosed in U.S. Pat. Nos. 4,231,042 issued to R. H. Turrin on Oct. 28, 1980 and 4,246,584 issued to A. R. Noerpel on Jan. 20, 1981.
The problem remaining in the prior art is to provide wide bandwidth hybrid mode feeds which are simpler to fabricate than prior art type feeds with wide bandwidth and also provide negligible reflection and generation of unwanted modes over bandwidths in excess of two octaves.